The present invention relates to the general field of gas turbine casings, and more particularly holding casings of gas turbine fans for aeronautical engines.
In a gas turbine aeronautical engine, a fan casing fulfils a number of functions. It defines the incoming air stream to the engine, supports abradable material with respect to the tip of the fan vanes, supports an optional structure for sound wave absorption for acoustic inlet treatment of the engine and incorporates or supports a holding shield. The latter comprises a trap for catching debris such as ingested items or fragments of damaged vanes thrown out by centrifuge action to prevent them from passing through the casing and reaching other parts of the aircraft.
Making a casing for holding a fan made of composite material has already been proposed. Reference could be made to document EP 1 961 923 which describes the production of a casing made of composite material of evolutive thickness, comprising the formation of a fibrous reinforcement by superposed layers of a fibrous texture and densification of the fibrous reinforcement by a matrix. According to this invention, the fibrous texture is made by three-dimensional weaving with evolutive thickness and is wound in several superposed layers onto a mandrel having a central wall of profile corresponding to that of the casing to be manufactured and two lateral flanges of profile corresponding to those of the external flanges of the casing. The resulting fibrous preform is held on the mandrel and impregnation by resin is completed under vacuum prior to polymerisation. The winding on a mandrel of a woven texture of evolutive thickness as described in this document directly gives a tubular preform having the preferred profile with variable thickness.
In practice, the resin impregnation step conducted under vacuum requires a supple envelope (or liner) to be applied to all the fibrous reinforcement, and especially at the level of the flanges of the reinforcement which will later form the external flanges of the casing. A difference in pressure is then set between the exterior and the space delimited by the mandrel and the liner containing the fibrous reinforcement. The injection of resin into this space can then start.
During this step, it was noted that setting the vacuum tends to generate tension in the layers of fibrous texture positioned at the level of the angles of flanges between the flanges and the central wall of the mandrel, this tension setting causing the fabrics to unstick at the origin of resin compaction and mass defects between the layers.